Organic electroluminescence (hereinafter also referred to as “organic EL”) display devices have in recent years attracted attention as flat panel displays which are excellent in terms of low voltage drive, all solid state, high speed response, self-emission, etc. An organic EL display device includes an organic EL substrate on which organic EL elements are formed, and a sealing substrate which faces the organic EL substrate and seals the organic EL elements. The organic EL substrate has, for example, the following structure: thin film transistors (hereinafter also referred to as “TFTs”) are formed on a glass substrate; a planarization film is formed for planarization and insulation of a surface of the glass substrate on which the TFTs are provided; and lower electrodes, an organic layer, and upper electrodes are successively formed, on top of one another, on the planarization film.
Organic EL display devices are roughly divided into the bottom emission type which allows light to be extracted through the side on which the lower electrodes are provided (see FIG. 44), and the top emission type which allows light to be extracted through the side on which the upper electrodes are provided (see FIG. 45). In a bottom emission type organic EL display device, the lower electrode is a transparent electrode of ITO etc., and the upper electrode is a light reflective metal electrode. On the other hand, in a typical top emission type organic EL display device, the lower electrode is a light reflective metal electrode, and the upper electrode is a semi-transparent electrode.
In recent years, on the mobile telephone market, there has been an increasing demand for display devices which can display an image on both front and back sides thereof, i.e., a primary screen (main display) and a secondary screen (sub-display). It has been desired to provide thin and light double-sided display devices having high display quality. In addition to the mobile telephone market, in the fields of display devices (mobile viewers) included in mobile devices, such as multimedia players etc., large-size information bulletin boards (digital signage), etc., the use of double-sided display devices has been expected to increase the range of design or expression or to provide the space saving effect.
Double-sided display devices currently in practical use have two display panels with their backs facing each other. Therefore, there is a limit on a reduction in thickness and weight of the double-sided display device, and at the same time, the use of two display panels is disadvantageous in terms of cost. Therefore, it is desirable that double-sided display be performed using a single display panel.
As a display device which performs double-sided display using a single display panel, a light emitting type display device is preferable. Attempts have been made to provide an organic EL display device which can perform double-sided display.
PATENT DOCUMENT 1 describes a double-sided organic EL display device in which bottom emission type organic EL elements and top emission type organic EL elements are formed, one on top of the other, on a single pixel circuit.
PATENT DOCUMENT 2 describes an organic EL display device in which each pixel region includes a first region which emits light downward and a second region which emits light upward.
PATENT DOCUMENT 3 describes an organic EL display device in which TFTs, a planarization film, a reflection film, lower electrodes, an organic EL layer, and upper electrodes are successively formed, on top of one another, on a glass substrate, and the reflection film has an opening in a light emission region or the reflection film is provided in an island-like pattern in a light emission region.